1. Field of the Invention
This invention generally relates to continuous tank-type glass melting furnaces and, more particularly, to the construction of such furnaces having reduced waist sections interconnecting the melting and conditioning zones thereof.
2. Description of the Prior Art
In a continuous, tank-type glass melting furnace, raw batch material and scrap glass or cullet are charged into one end of the furnace and molten glass is removed from its other end. The glass, in moving through the furnace, passes successively through melting, refining, conditioning or cooling and working zones which are contiguous with one another. Heat is applied over the upper surface of the bath of glass in the melting zone for reducing the newly added materials to a molten state and integrating them into the flowing molten mass, and the molten glass mass is refined and cooled to a point where it can be removed from the working zone in a continuous ribbon.
Conventionally, one such type of furnace is constructed with a lower tank section covered by an independently supported roof section wherein the central region of the tank section is provided with a reduced waist which provides a passage intermediate the ends of the tank. This construction, in effect, forms separate melting and conditioning or cooling tanks interconnected by a passage that is slightly smaller in width than the tanks so that the melting operation can be performed to best suit conditions in the melting tank without affecting the molten glass in the working tank. The size of the waist and, of course, the passage, is determined by the output and the operating conditions of the melting furnace. The tanks are of constant depth and the width of the tanks is constant except for the waist section where the side walls are inset to provide the narrow region interconnecting the melting zone to the conditioning zone for improving the homogenity of the glass in the working zone.
It is common practice to cover the melting and conditioning sections with separate, suitably sized sprung arch roofs wherein the central portions of upright walls closing the ends of the melting and conditioning sections at the waist, are supported on sprung arches. As is known, the sprung arches spring from skews set on heel plates firmly attached to buck stays forming part of the furnace superstructure. Normally the arches are built up of individual tapered like-size bricks by dipping the individual bricks in a siliceous mortar and setting them in place on a form supported by scaffolding. When the form on which the arches have been laid during construction is removed, the slight elastic yielding of the buck stays and the tie rods together with the bedding in of the contacting surfaces of adjacent bricks against each other, allows the crown of the arches to drop slightly and small gaps to open between the lower ends of the bricks. Accordingly, the portion of the walls supported by the sprung arches will also drop.
It is the usual practice in glass melting furnaces to construct the walls and arches of silica bricks which are quite heavy, i.e. a brick 9" (229 mm).times.6" (152 mm).times.3" (76 mm) weighs approximately 9 pounds. At this point, it should be noted that the wall portions supported by the roof arches are on the order of fourteen feet (4.27 m) wide, have a height of about ten feet (3.05 m) and are approximately one and one-half feet (0.46 m) thick. Conventionally, these wall portions are built up of a number of individual courses of refractory brick, and wall portions of sufficient size weigh in the neighborhood of 12,000 (5443 kg.) pounds, which would be supported by the roof arches.
Also, it should be noted that refractory bricks, formed of silica, exhibit their maximum thermal expansion at low temperatures. Newly constructed furnaces, as above-described, are often very damp and they are slowly dried out over a period of time (ten to twelve days) by temporary burners such as salamanders placed at convenient locations about the furnace. During this time, the temperature throughout the furnace including the walls and the arch roofs will not be uniform. Accordingly, it may be found that the bricks, when the furnace is heated-up, expand at different times or rates and that the walls and arch roofs may warp during the drying out period by rising unequally in one part or another with respect to the plane of symmetry of the tank. Consequently, as heating occurs and the bricks dry out, some may become sufficiently loose in the roof arches to drop therefrom, thus permitting the supported portion of the upright closure walls to collapse into the interior of the waist section or, if the arches do not collapse, the walls to buckle and rupture.